1. Field of the Invention
The present invention relates to a light sensing circuit capable of measuring an optical amount, a backlight control apparatus having the same, and a liquid crystal display (LCD) device having the same, and more particularly, to a light sensing circuit capable of lowering a dependency on a temperature change without using a resistor, a backlight control apparatus having the same, and an LCD device having the same.
2. Discussion of the Related Art
Generally, an LCD device serves to display a desired image by controlling an amount of light that passes through a liquid crystal layer by controlling an arrangement of a liquid crystal molecule having a refractivity anisotropy using an electric field.
The LCD device consists of an LC panel, and a backlight positioned at a rear side of the LC panel for irradiating light into the LC panel. The LC panel for substantially forming an image includes a lower substrate, an upper substrate, and an LC layer positioned therebetween. The lower substrate is a thin film transistor (TFT) substrate on which a TFT and a pixel electrode are formed. The upper substrate is a color filter substrate on which a black matrix (BM), a color filter layer, and a common electrode are formed. A polarizer is attached onto an outer surface of the TFT substrate and the color filter substrate. A driving circuit portion is provided at an edge of the lower substrate, thereby respectively supplying a signal to the TFT, the pixel electrode, and the common electrode formed at the lower substrate.
The backlight includes a lamp for substantially emitting light, a reflection plate for enhancing an optical efficiency by reflecting light emitted from the lamp, and an optical sheet for uniformly introducing light emitted from the lamp into the LC panel.
In the conventional LCD device, the backlight may not generate a high brightness at a dark place. However, the conventional backlight has been constructed so as to maintain a brightness constant regardless of a peripheral brightness, thereby wasting power.
To solve the problem, a technique for detecting a peripheral brightness of the LCD device and controlling an optical amount according to the detected brightness has been proposed.
FIG. 1 is a circuit diagram showing a light sensing circuit for an LCD device in accordance with the related art. As shown, the light sensing circuit includes a MOS-transistor TFT 11 installed in the LC panel, for detecting an optical amount thereby generating a voltage based on the detection result; and a resistor R 11 connected between a source terminal and a ground terminal of the MOS-transistor TFT 11, for sensing an optical amount by the MOS-transistor TFT11. An operation of the light sensing circuit will be explained.
The MOS-transistor TFT 11, an amorphous-silicon type TFT is installed in the LC panel. The MOS-transistor TFT 11 includes a gate, and source/drain separated from each other based on the gate. A voltage (VH) is supplied to the source, and a bias voltage (VL) is supplied to the gate. The drain of the MOS-transistor TFT 11 is connected to the ground terminal through the resistor R 11.
A current of the amorphous-silicon type TFT becomes different according to an optical amount. When an amount of irradiated light is large, a current intensity is increased. That is, when an amount of light irradiated into the MOS-transistor TFT11 is increased, a voltage output through the drain is increased.
An inverter driving controller (not shown) detects change of a voltage output from the MOS-transistor TFT 11 by an optical amount detecting terminal (Vd) connected between the drain and the resistor R 11 of the MOS-transistor TFT11. Then, the inverter driving controller detects a peripheral brightness of the LCD device, thereby controlling a brightness of the backlight. For instance, when the peripheral brightness of the LCD device is dark, the brightness of the backlight is lowered thus to operate the backlight in a saving mode.
However, the MOS-transistor TFT 11 and the resistor R 11 are influenced by temperature. When the MOS-transistor TFT 11 and the resistor R 11 are operated in different temperatures, an optical amount variation is not precisely detected. Furthermore, the MOS-transistor TFT11 disposed in the LC panel and the resistor R 11 disposed at the driving circuit portion are influenced by different temperatures. Accordingly, an optical amount variation is not precisely detected thus to lower a reliability of the light sensing circuit.